Hazards to Navigation: Fog

By Andrew Spaulding

With 80 degree temperatures and boats going over the wall at a furious pace, you’d think that spring time is over and we are well on our way to summer. However, current conditions are perfect for fog formations in the afternoons and evenings. So venture out prepared and take the initiative to leave Crowley’s dock a bit early to make it to the harbor before the evening fog comes up.

What is fog? There are many types of fog, but for the most part they are very similar. Technically, fog is a type of stratus cloud that stays very close to or on the ground. Fog begins to form when water vapor condenses into tiny water droplets in the air and normally occurs at a relative humidity near 100%. This is achieved by either adding moisture to the air or dropping the ambient air temperature.

Advection fog occurs when moist air passes over a cool surface and is cooled which is what happens here in Chicago before the lake warms up. When the sun goes down the warm, humid air is no longer being warmed, and the cool air off the lake chills the air, causing the water vapor in it to condense into water droplets. Wham-O, we’ve got fog. The tough part about this is it coincides with the sun going down which can make for an interesting trip back to the harbor.

What to do? Be prepared. If you’re going to be out after sunset be ready for fog, particularly if the wind is light or out of the east. On the way out, make sure that your navigation electronics are working and you have compass headings and distances written down so that you can “dead reckon” if necessary.

Once the fog rolls in, proceed at a cautious speed making the appropriate sound signals for your vessel. A power boat underway is required to make one prolonged blast (4-6 seconds) of their fog horn every two minutes. A power boat stopped on the water is required to make two prolonged blasts (not more than 2 seconds between them). Most other vessels including ones with restricted maneuvering, fishing, sailing, towing or pushing are required to make three blasts in succession: one prolonged blast followed by two short blasts. Also, don’t navigate directly from buoy to buoy. Lots of people do this and it can lead to a pile up (literally) at the buoy. Instead, navigate a bit away from the buoy you are aiming for, on the correct side of course.

Thanks to Mike Hamernik and the Chicago Weather Center Blog (click here to view) for the article idea.